The present invention relates to an improvement of a direct modulating FSK transmitter for MCA (Multi Channel Access) and in particular to a circuit construction for eliminating instantaneous frequency deviation in an FSK (Frequency Shift Keying) transmitter wherein a data signal is transmitted by means of direct modulating method.
In an FSK transmitter of direct modulation type for MCA, a VCO (voltage Controlled Oscillator) for direct modulation is controlled by a frequency synthesizer having a PLL (Phase Lock Loop) so as to oscillate with an object frequency of a predetermined channel. Thus, when the frequency synthesizer is set by a channel setting instruction and the PLL operates, after its response time lapses, the frequency of said VCO arrives at the object frequency of the predetermined channel. A transmitting power amplifier of a following stage is let to be off because the frequency is not set correctly yet until it arrives at this object frequency. A portion of a frequency divider and a phase detector in the frequency synthesizer is formed by an LSI (Large Scale Integrated) circuit and when the frequency arrives at the object frequency and the PLL becomes in synchronized condition, the frequency synthesizer produces a lock-in signal indicating completion of synchronized condition. Accordingly, when the lock-in signal is provided, since the VCO has been set to the object frequency already, the transmitting power amplifier becomes ON and a modulating signal is inputted to the VCO to start transmission of data.
However, when the transmitting power amplifier is caused to be ON, the following problem happens.
That is, an output of the VCO is connected to the transmitting power amplifier through a buffer amplifier and ON/OFF of a transmitting power is done by ON/OFF of a power supply of the transmitting power amplifier. An input impedance of the transmitting power amplifier is varied by ON/OFF of the power supply thereof. Variation of this impedance influences a load impedance of the VCO through a feedback capacity of the buffer amplifier so as to make it vary and thus the frequency of the VCO varies. However, since an output frequency of the VCO is controlled by the PLL synthesizer, if instantaneous frequency variation happens, correcting action is done automatically thereafter.
In this way, channel setting is done, so that correct frequency is set but when the transmitting power amplifier becomes ON, said output frequency is instantaneously pulled down immediately thereafter and then is pulled up to the correct frequency again by means of response operation of the PLL.
In other words, there are such drawbacks that a signal having frequency error is outputted instantaneously and thus transmission of data must be waited until the output frequency is pulled up to the correct frequency. In particular, with respect to a FSK signal, since information is transmitted in frequency domain, frequency error influences the information directly. Of course, since range of this frequency deviation becomes small if design for making the feedback capacity of the buffer amplifier be greatly small is done, influence of the frequency error can be made small but since the impedance of the feedback capacity becomes large as treated frequency becomes high even if the feedback capacity is small, it is difficult to make said influence small.
In view of the above mentioned present situation, an object of the invention is to provide a direct modulating FSK transmitter for MCA having a simple correcting circuit for making a carrier frequency not deviate or its deviation lay within an allowable range even if the transmitting power amplifier becomes ON.
In order to attain the above object the subject matter of the invention is to construct a direct modulating FSK transmitter for MCA so as to cancel frequency deviation of a transmission signal by applying an ON/OFF control signal to a voltage controlled oscillator for direct modulation in the direct modulating FSK transmitter for MCA wherein after a PLL frequency synthesizer is locked to an object frequency of a predetermined channel, the modulation signal is applied to the voltage controlled oscillator for direct modulation in the PLL frequency synthesizer and the transmitting power amplifier is caused ON in response to the ON/OFF control signal to output the transmission signal outputted from the voltage controlled oscillator through the transmitting power amplifier.
In the invention the voltage controlled oscillator may be constructed so as to have a variable capacity diode to which the transmission signal is applied, and so as to add the ON/OFF control signal to the variable capacity diode through a registor.
In the transmitter, the variable capacity diode is used as an element for making the frequency of the VCO variable and PLL operation is obtained by applying a frequency control voltage from a phase detector and the transmission signal to it. So, since a load capacity of the VCO increases and a transmission frequency becomes low instantaneously by making the transmitting power amplifier ON in an ordinary construction, frequency deviation owing to ON of the transmitting power amplifier can be cancelled by applying the ON/OFF control voltage to the variable capacity diode to correct it.
A direct modulating FSK transmitter for MCA of the invention is constructed so that the frequency deviation of a transmission signal owing to ON of the transmitting power amplifier can be cancelled by applying the ON/OFF control signal to the VCO for direct modulation on the basis of the above mentioned idea.